The present invention refers to a mobile working machine for treatment of a ballast bed or subgrade for a track comprised of rails fastened to ties, and in particular to a working machine of the type having a machine frame supported by undercarriages for movement in an operating direction along a track, and a track lifting unit which is vertically adjustably mounted to the machine frame between two undercarriages for lifting the track.
A mobile working machine of this type is known from U.S. Pat. No. 4,479,439 and is used for replacement of a track ballast bed while laying a protective layer for the subgrade at the same time. The machine frame which travels continuously along the track in operating direction is supported at its axial ends by undercarriages and coupled to further freight cars. During travel from site to site, one of the undercarriages of the machine frame is lowered onto the track so as to run securely on the track while the operating range of the vehicle is substantially lengthened at the working site by lifting the undercarriage off the track to thereby effectively increase the length of the construction section. A track lifting unit is arranged approximately central between the terminal undercarriages and is vertically adjustably mounted to the machine frame for lifting the track in order to create beneath the ties sufficient space for track bed working units comprised of an excavating chain for exposing the subgrade as well as a vertically adjustable planing unit for leveling and compacting sand and ballast laid upon the subgrade. The track lifting unit includes laterally pivotal flanged rollers which engage from both sides under the rail head for lifting and continuously holding the railroad track at a desired level. When encountering changes in the rail cross sections, e.g. during travel over of a rail joint, the locking engagement of these flanged rollers with the rail head may be compromised, thus adversely affecting a permanent and secure support of the rail.
U.S. Pat. No. 3,179,062 discloses an under-track device for treating a track bed from beneath a railroad track and includes a frame which advances in wedge-like manner longitudinally in direction of the track. This device is supported via track-type conveyors, which are driven by a hydraulic motor and vertically adjustably mounted with the frame, upon the ballast surface or subgrade surface which is planed by clearing tools located at the forward end of the device. The vertical support and spacing of the railroad track from the ballast bed is attained by two endless tracks which extend longitudinally in direction of the track and are arranged in the area below the rails on the upper side of the frame of the device. These endless tracks press against and run on the bottom of the ties while the device travels through the track bed. Arranged above the device is a vehicle which travels along the track and is equipped with a power source and a control unit and is moved together with the device via a link mounted on the longitudinal side of the machine frame, with the weight of the vehicle being also transmitted via the endless tracks onto the track bed. Such a device requires a very high lifting of the track and thus causes inadmissible flexures of rails in particular when lifting heavy rails.
A similar device for planing a ballast bed surface from beneath a railroad track is described in Canadian patent no. CA 523,843 and includes a carriage-like slide in form of a horizontal plate which is pushed and pulled beneath the ties by a machine running on a track in a wedge-like manner between track and ballast bed. In order to lift the track from the ballast bed, the top of the slide is provided with a large number of rolls which are arranged in two rows and carry two endless chains longitudinally in direction of the track and run in the area beneath the rails upon the tie bottom.